The adsorption of polypeptides and proteins at fluid-fluid interfaces is fundamental to a number of industrial applications ranging from food processing (Faergemand et al., 1997 and references cited therein) and biphasic catalysis (Cascao-Pereira et al., 2003a) to oil recovery (Mohammed et al., 1993). Adsorbed protein layers confer mechanical strength to fluid interfaces, thus altering their properties. In oil-water emulsions, the presence of a protein layer having mechanical strength affects both the ease of initial droplet disruption (Williams et al., 1997) and subsequent emulsion stability during storage (Dickinson et al., 1988). Similarly, in foams, the presence of an interfacial protein layer possessing mechanical strength increases foam stability in the medium to long term (Cascao-Pereira et al., 2003b, Cascao-Pereira et al., 2003c).
The viscoelasticity of a protein layer adsorbed at a fluid-fluid interface can be determined directly (Jones and Middelberg, 2002a, Jones and Middelberg, 2002c) and the presence of a protein layer capable of transmitting force in the interface predicts emulsion stability under conditions where predictions based on interfacial tension fail (Jones and Middelberg, 2003). Furthermore, the ability to transmit force in a fluid interface is not an exclusive property of proteins, but short (11-25 residue) peptides can also form force-transmitting networks at fluid-fluid interfaces (Jones and Middelberg, 2002b). Like proteins, such peptides self-locate to a fluid interface from bulk solution and may form nanostructured networks with defined mechanical properties. Such peptide networks may be useful in the formation of emulsions and foams and could also be useful in coatings or in the formation of drug delivery agents.
However, in a number of situations, stabilization of a fluid-fluid interface, such as formation of a stable emulsion, foam, coating or drug delivery agent may be required for a particular reason but subsequent stabilization of the interface is not required. Alternatively, there may be a requirement to delay formation of a stabilized fluid-fluid interface, thereby delaying formation of a stable emulsion, foam, coating or drug delivery agent. In other applications there may be a requirement to stabilize and destabilize a fluid-fluid interface, for example in an emulsion, foam, coating or drug delivery agent, a plurality of times. An emulsion may be destabilized by the addition of a demulsifier and a foam may be destabilized by the addition of a defoamer. However, the use of traditional demulsifiers and defoamers can be expensive and impose additional complexity and cost on a method. Furthermore, once added, a traditional demulsifier or defoamer causes the breakdown of an emulsion or foam or prevents an emulsion or foam forming but its effects may be difficult to reverse or remove after addition as the chemical composition of the interface is altered by the addition of agents that compete for interfacial space. There is a need for a method which allows modulation of the stabilization of a fluid-fluid interface, such as in an emulsion, foam, coating or drug delivery agent, to allow control of formation, dissipation and the strength or stability of the emulsion, foam, coating or drug delivery agent formed.
The present inventors have now found that interfacial characteristics, such as force transmission, of a self-assembled, force-transmitting peptide network may be modulated allowing the properties of a fluid-fluid interface, such as in an emulsion, foam, coating or drug delivery agent, formed with the peptide network to be manipulated in a predictable manner thus allowing the peptide networks to be formed and dissipated in response to particular stimuli or allowing the strength and/or elasticity and/or rate of formation of the peptide network to be manipulated by a particular stimulus.